1. Field of the Invention
The present invention relates to an apparatus for optical detection and analysis and in particular an apparatus for identification of a selected optical system.
Light beams will interact with any object in such a way that some energy will be diverted or converted. Depending on whether the object consists of refractive or reflective interfaces such interactive mechanisms will include scattering, reflection and refraction, all of which differ in efficiency within any given optical system depending on the light wavelength and the physical characteristics of the object.
For example, the phenomenon of scattering is what underlies the operation of known optical range-finding systems such as a LIDAR (Light Distancing and Ranging), which is an optical equivalent of a RADAR system, or a Laser Range Finder system (LRF). In LIDAR/LRF systems a light pulse is sent out from the system which then waits for any return signals. By timing the interval between transmission and reception the distance to the target object can be calculated. The operation of such systems is covered in “Introduction to Radar Systems” by M. I. Skolnik (McGraw Hill).
A problem with existing LRF/LIDAR systems is that they provide no information about a target system other than how far away it is. Target identification is crucial in, for example reconnaissance and surveillance situations and therefore it would be desirable for an optical range-finding system to also be capable of identifying the type of optical system being ranged.
Positional analysis of optical systems is also important in the production of complex optical systems, such as microscopes, telescopes etc., from the point of view of quality control. Currently, the position of optical components within such systems must be inferred from the optical performance of the system measured via interferometers or Modulation Transfer Function (MTF) equipment. It would therefore be desirable to have a device capable of directly measuring the position of optical components within a built up optical system.
2. Discussion of Prior Art
It is possible to exploit one of the other interactive mechanisms mentioned above in order to obtain more information about a target system. If an optical system is illuminated by a light source some light will be reflected back towards the light source—this is the phenomenon of RetroReflection. If a light detector is used in conjunction with a light source then the presence of an optical system can be detected.
The simplest example of a system in which RetroReflection occurs is an everyday simple mirror. Another example of a RetroReflection generator is the “Cat's Eye” system used on roads. In this device light, from car headlights, is focused onto the surface of a reflector and retroreflected out again.
The presence of a “mirrored” reflector is not necessary for RetroReflection to occur. Whenever an optical wavefront encounters a change in refractive index, it changes its velocity slightly since the speed of light is different in different materials. If the wavefront encounters a refractive surface at an angle the net result is that the transmitted beam of light bends, the process of refraction. However, this simple view of the interaction takes no account of the imperfection of the interface between the two materials. In much the same way that electrical cables need to be impedance matched into their terminating loads then light waves need to be impedance matched across refractive boundaries. For the case of a light ray which has normal incidence at a refractive boundary it was shown by Fresnel that the refractive indices, n1 and n2, of the materials on each side of the boundary cause a certain proportion of the incident light to be reflected in the ratio:   r  =                    (                              n            1                    -                      n            2                          )            2        /                  (                              n            1                    +                      n            2                          )            2      
For the case of a vacuum/glass transition (n1=1.000; n2=1.5) this surface reflection ratio is 4.2% and there will therefore be RetroReflection generation. In other words if a glass or plastic system is illuminated by a light source there will still be RetroReflection which can be exploited to obtain information about the target.
Most optical systems will, however, have some sort of structure. For example, binoculars have an internal structure consisting of a series of lenses and prisms all of which will RetroReflect. Since each optical surface will RetroReflect some of the incident irradiating light there will be multiple retro-reflected light signals which will vary between different optical systems. Therefore, different optical systems will have different “optical signatures” and it should be possible to analyse the signature to determine the characteristics of the target optical system. It should be noted that the term “optical system” need not refer to a system that consists of a series of glass lenses—any object that has a series of reflecting surfaces, be they “mirrored” surfaces or glass/air transitions or otherwise, should be considered as an optical system. The human eye, for example, will also generate an optical signature.
However, the method by which the “optical signature” is extracted from the reflected light will be crucial. This is because even in the simplest optical systems the returning combination of wavefronts is likely to be very complex due to multiple internal reflections. If a highly coherent light source, such as a laser, is used then the reflected wavefronts will be able to vectorially add and subtract resulting in an interference pattern (this is due to the fact that spatial coherence means that the reflected wavefronts bear a fixed relationship to one another and are therefore able to optically interfere). Attempting to use such an optical interference pattern as the “optical signature” will result in problems due to the short wavelength of light. Firstly, the components of optical systems do not normally require to be assembled to interferometrically close tolerances and so optical systems produced successively on the same production line would have differing signatures. Secondly, the wavelength of light is much smaller than the path differences induced by the passage of a light beam through the atmosphere under normal meteorological conditions and any intereference pattern will therefore be swamped by fluctuations caused by the atmosphere.
Although atmospheric turbulence will rapidly wreck the structure of an optical interference pattern the temporal coherence of a light beam is fairly well insensitive to atmospheric effects. This is because the effective temporal coherence length of photon bunches within a light pulse is much greater than the path length variations due to density differences. Therefore, there will be an “optical signature” associated with the timing of individual groups of photons arriving at a detector that represent the RetroReflection from each surface of an optical system.
In order for the structure of the target optical sight to be resolved the light pulses generated by the light source will have a certain size limit depending on the target optical system. Bearing in mind that light can travel 30 centimetres in one nanosecond then to resolve components that are separated by 15 centimetres (outward and return journey of the light beam means this distance is effectively 30 cm) the source will have to generate light pulses which are equal to or less than one nanosecond in duration. Light pulses of approximately such duration or less are hereinafter referred to as “ultrashort” pulses. Note: Since optical components are often much closer then 15 centimetres, much shorter pulses are required, of the order of tens of ferntoseconds.